A corrugated sandwich panel is formed in such a manner that a vertically corrugated inner sheet as a reinforced member is inserted between upper and lower polyethylene sheets and is then bonded to the bottom and top surfaces of the upper and lower polyethylene sheets, respectively. So, the corrugated sandwich panel exhibits a substantially excellent strength. If such the corrugated resin sandwich panel is produced in great quantities, it will be applicable in various kinds of industrial fields. For example, it can be applied to water storage tanks for general buildings, apartments, and farms and to sewage storage tanks for barns.
Various kinds of water storage tanks have been made of fiber reinforced plastic (hereinafter, referred to as “FRP”) or glass fiber reinforced plastic (hereinafter, referred to as “GFRP”). When the FRP or GFRP is used for large-sized water storage tanks, however, it is not strong enough to resist against its substantially high water pressure. Therefore, it is found that it is impossible to manufacture the large-sized water or sewage storage tanks by use of the conventional FRP or GFRP.
On the other hand, a polyethylene resin sandwich panel, which is provided with a vertically corrugated inner sheet that is formed as a reinforced member between upper and lower polyethylene sheets in such a manner as to be bonded to the bottom and top surfaces of the upper and lower polyethylene sheets, respectively, exhibits much higher strength than the conventional FRP or GFRP. However, a technique where the inner sheet is bonded between the upper and lower polyethylene sheets has not been widely put into practical use, yet.
A conventional method for making the polyethylene resin sandwich panel includes the steps of molding upper and lower flat polyethylene sheets, additionally molding a vertically corrugated inner sheet, and bonding the inner sheet between the upper and lower sheets in a manually operated manner. Such the conventional method makes the operating efficiency remarkably lowered, which results in the reduction of the productivity. This of course finds it impossible to make the polyethylene resin sandwich panel in great quantities.
The present inventors have made various studies to solve the above problems and as a result, they have found that the polyethylene resin sandwich panel having a reinforced inner sheet is made in a single process, exhibiting a substantially high strength by way of its excellent bonding power. In addition, it can be produced in great quantities.